<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style>
        .arrow{
            display: inline-block;
            border-top: 10px solid black;
            /*border-right: 10px solid transparent;*/
            border-bottom: 10px solid transparent;
            border-left: 10px solid transparent;
            /*想要哪边箭头，直接把另外三边的颜色改成透明:transparent.*/
        }
    </style>
</head>
<body>
    <div class="arrow"></div>
</body>
</html>